This invention relates to means for reducing noise generated by operation of a hydraulic pump such as a displacement-type rotary pump designed for axial flow.
Liquid-pumping machinery will generate a flow ripple measured as a fluctuation in the developed pressure. It is necessary to control the magnitude of this pulsation in order to effectively operate a fluid-pumping system without undue noise, vibration and eventual equipment failure.
Low-noise operation of liquid-pumping systems requires methods to control pressure pulsations generated by the pump. The practice today is to use acoustical devices which fall into one or more of the following categories:
1. Expansion chambers, or large volumes. These work well over a broad frequency range; however, they necessarily involve large pressure vessels and tend to be very heavy and expensive.
2. Gas-filled accumulators. These provide good noise reduction in the lower frequency ranges; however, they have the disadvantage of requiring maintenance, and they tend to be expensive.
3. Helmholtz resonators. These provide effective attenuation above their resonant frequency but fall short at low frequency due to size limitation, and they too are expensive.
Generally, most available methods tend to provide more attenuation than is necessary and become too costly for competitive applications. Thus, unfortunately, noise problems fail to get resolved.
Pumps of the character indicated find important application to operation of hydraulic elevators, as for example in a hotel or apartment house where noise must be kept to an absolute minimum, in consideration of those residents whose accommodations are near an elevator shaft. The pump in such a system is immersed in the sump, and of course any pulsation in pump operation will be transmitted directly and hydraulically to the plunger or other end-use actuator of the system; operating pressures are in the order of hundreds of pounds per square inch, and further noise can be transmitted to support structure via piping which connects the pump to the end-use actuator or actuators. To apply effective noise-reduction techniques to the piping calls for custom design, which may be and often is economically wasteful, being the product of empiricism under the direction of inadequately skilled installation personnel. And once noise is tolerated in the piping, the problem of noise reduction involves both liquid-borne noise as well as structure-borne noise, propagated via the material of the piping. High-level structure-borne noise very often can result in high-level airborne noise.
Generally, a three-screw pump of the variety commercially available from the Imo Pump Division of Imo DeLaval Inc., Monroe, N.C., is to be desired for applications of the character indicated. This three-screw pump is a positive rotary pump with axial-flow design. The central one of three screws is motor-driven, and the two further screws are idlers meshing with diametrically opposed portions of the driven central screw, the idlers acting as sealing elements that are rotated hydraulically by the fluid being pumped. The rolling action which characterizes idler reaction to the hydraulic fluid accounts for a continuous-non-pulsating axial flow which, in the case of an end-use elevator, results in a smooth elevator ride, but the smooth ride cannot conceal the presence of hydraulically conveyed noise, which is always objectionable, even if markedly reduced as compared with delivery from other kinds of pump systems.